1. Field of the Invention
The present invention relates to a four-wheel hydraulic drive system for a working vehicle including a hydraulic pump, a first hydraulic motor fluidly connected therewith so as to drive first driving wheels, and a second hydraulic motor for driving second driving wheels, wherein a braking force applied on the first driving wheels by braking devices can extend to the second driving wheels.
2. Related Art
There has been a conventional four-wheel drive working vehicle provided with a hydraulic transmission including a hydraulic pump driven by an engine and a hydraulic motor fluidly connected with the pump for driving either pair of front or rear wheels (usually, rear wheels). Such a conventional four-wheel drive vehicle has been provided under the bottom of the midsection thereof with a transmission shaft for transmitting power from the hydraulic transmission to the other pair of wheels (usually, front wheels).
As compared to a two-wheel drive vehicle, the four-wheel drive vehicle easily creates a stronger propelling force so as to be effective in climbing a hill or working in a paddy field.
In the conventional construction, the peripheral speed of the front wheels is normally set higher than that of the rear wheels so as to minimize the cornering radius of the vehicle in field. The problem has arisen that the front wheels tend to be dragged during a small radius cornering so as to prevent the vehicle from smooth cornering.
During usual travel on a flat road, such as asphalt pavement or the like, the same problem arises that the difference of peripheral speed between the front wheels and the rear wheels causes the front wheels to slip or be dragged and to wear, even if the vehicle travels in a substantially straight direction. The tendency that the front wheels are dragged is increased in proportion to the travelling speed of the vehicle. Furthermore, driving of four wheels wastes power. Therefore, a typical driver on a flat road has shut-off power transmission to the front wheels, so as to drive only the rear wheels. In other words, the driver has caused the vehicle to travel in the two-wheel drive mode during usual travel on a road. However, when the rear wheels are braked in the two-wheel drive mode, frictional resistance is only generated between the rear wheels and the road, such that the problem has arisen that the braking distance becomes longer than that in the four-wheel drive mode; wherein the braking force to the rear wheels extends to the front wheels such that frictional resistance is generated between all four wheels and the road.
Additionally, there has been the problem that a working machine of a type equipped on the bottom of the midsection of the vehicle body must be designed to avoid the intervention with the above-mentioned transmission shaft disposed under the same.